Observational analyses are performed to examine the roles of remote and local
forcing in the evolutions of the extreme U.S. summer heat wave-drought cases of
1980 and 1988. At early stages, both events are associated with anomalous
stationary wave patterns. Wave activity flux analyses suggest that in the 1980
case anomalous wave activity propagates southeastward from an apparent source
region to the south of the Aleutians. The flux pattern is more complex in the
1988 case but suggests two possible source regions, one over the central North
Pacific to the north of the Hawaiian Islands and a second located over the far
western Pacific. The 1988 analyses show no anomalous wave propagation out of
the eastern tropical Pacific, although this result does not necessarily
preclude a role for tropical forcing in generating the anomalous wave train.

In both cases the anomalous wave trains and associated wave activity fluxes
become very weak by early July, indicating that remotely forced anomalous
stationary waves are unlikely to account for the later stages of the heat
wave-droughts. This leads us to examine whether these events were enhanced or
prolonged by changes in the local surface energy budget associated with
reductions in evapotranspiration (ET) over the drought regions. Water vapor
budgets show a systematic decrease in monthly mean ET from June to August
during both events. Comparisons with nondrought summers support the idea that
by late summer ET rates in both events are anomalously low. Estimated
reductions in surface latent heat fluxes relative to the control years are
approximately 50 W m-2 in 1980 and 20 W m-2 in 1988, with
implied increases in sensible heating of similar magnitudes.

Overall, the results indicate the importance of both dynamical forcing from
remote sources and anomalous local boundary conditions in accounting for the
two extreme heat wave-drought events. The relative importance of these factors
varies significantly during the evolution of the events, with remote forcing
playing a predominant role at early stages and anomalous local boundary
conditions assuming increasing importance at later stages.